The present invention relates to an actuator capable of controlling a rotational position and a travel position according to the coil current and is applicable, for example, to the throttle control, ISC (Idle Speed Control) and variable intake control of an internal combustion engine.
The prior art will be described with regard to a throttle actuator for an automobile by way of example. The throttle actuator needs to move a moving member, e.g. a rotor, according to driving force (magnetomotive force) corresponding to the magnitude of an electric current. The present applicant has already proposed Japanese Patent Application Nos. 7-56686 and 8-140665.
An outline of the prior art will be described below by using FIGS. 1(1)-12(d). FIG. 12(a) is a front view (when the coil is not energized); FIG. 12(b) is a plan view; FIG. 12(c) is a side view showing the prior art by cutting the front view along the center line; and FIG. 12(d) is a front view (when the coil is energized). As shown in FIG. 12(a), reference numeral 1 denotes a yoke, and a coil 2 as a drive source is provided on the central upper portion of the yoke 1. A main air gap 3 is provided under the coil position in the center, and a rotating member 6 is rotatably fitted in the main air gap 3. The rotating member 6 has a magnet 5 secured to the peripheral edge of a rotor 4. It should be noted that the rotating member 6 rotates about a shaft (lying at the center position of the rotating member). Reference numeral 7 denotes a core.
Next, the operation will be described. In FIG. 12(a), when the coil 2 is not excited, magnetic flux from the north pole of the magnet 5 reaches the south pole via the yoke I, which lies on both sides of the magnet 5. Therefore, this state (state illustrated in the figure) is stable, and the illustrated state is maintained. When the coil is excited, the rotating member 6 is rotated by magnetic flux (FIG. 12(d)) from the coil 2 to move to the most stable rotational position and stops at this position. It should be noted that the above-mentioned Japanese Patent Application No.7-56686 is of the type in which counterforce is obtained by a magnetic spring. Although not shown, Japanese Patent Application No. 9-111857, for example, obtains counterforce by a magnetic spring as in the case of the above and uses an auxiliary mechanical spring only in the vicinity of a default opening position (a position where a correct throttle valve position is ensured even when the coil is brought into an unenergized state by a failure).
The prior art described above with reference to FIGS. 12(a)-12(d) requires a large amount of yoke material and also needs a large amount of magnet material because the magnet is provided on the entire circumference of the rotor 4. Consequently, the overall configuration becomes large in size. Although not described in detail, Japanese Patent Application No. 9-111857 uses a mechanical auxiliary spring in the vicinity of the default position and is therefore structurally complicated and costly.
The present invention was made to solve the above-described problems, and an object of the present invention is to provide a lightweight and low-cost actuator by reducing the weight through size reduction and also reducing the amount of magnet material used. Other objects of the present invention are to provide an actuator which has a stable and highly accurate default position, allows the default position to be set easily, which has a simplified structure, and requires a reduced amount of magnet material.
The present invention provides an electromagnetic actuator wherein a rotatable rotor and at least one magnetomotive force source are integrally incorporated through a magnetic path. In the actuator, an integral magnet magnetized with a north pole and a south pole, or separate magnets magnetized in opposite directions to have a north pole and a south pole, respectively, are provided on the peripheral edge of the rotor. In addition, three pole pieces are provided on the peripheral edge of an opening in which the rotor is provided, and each pair of adjacent pole pieces is connected by a connecting path. By virtue of the described arrangement, the connecting paths are less likely to be magnetically saturated, and thus the sectional area of the yoke can be reduced.
In addition, the present invention provides an electromagnetic actuator wherein a rotatable rotor and at least one magnetomotive force source are integrally incorporated through a magnetic path, and wherein an integral magnet magnetized with a north pole and a south pole, or separate magnets magnetized in opposite directions to have a north pole and a south pole, respectively, are provided on the peripheral edge of the rotor. In addition, three pole pieces are provided on the peripheral edge of an opening in which the rotor is provided, and each pair of adjacent pole pieces is connected by a connecting path. In the actuator, an outward projecting space is asymmetrically provided in the peripheral edge of the opening, and two of the pole pieces are provided at both extremities of the operating range of the rotor, wherein the lengths of portions of the two pole pieces that face the rotor are different from each other.